Modeling the Antenna Mode of the Transmon Qubit

We describe analytical and numerical studies of the spurious antenna mode of the transmon qubit. Through coupling to this mode, the qubit can become an efficient absorber of pair-breaking radiation. We describe a scheme to exploit this antenna coupling for the purposes of dark matter detection. Here, photons transduced from dark matter axions are converted to quasiparticles in a weakly charge-sensitive qubit. Ramsey-based charge tomography provides access to the quasiparticle parity of the qubit island. This approach provides access to the axion mass range from 10 GHz to 10 THz, a region of phase space that is difficult to access using existing methods.